refactor: split session handler modules

[thymikee]

Summary

Refactor the session handler family by extracting command-specific modules and shrinking the remaining routers.

• move inventory, state, observability, and replay handling out of session.ts
• split open preparation and replay runtime/healing into focused helper modules
• add focused regression tests for the extracted seams

Closes #267

Validation

• pnpm typecheck
• pnpm test:unit
• pnpm test:smoke
• pnpm format

[github-actions]

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Validate relaunch args before refreshing/resolving device

prepareExistingOpenCommand now calls refreshSessionDeviceIfNeeded (and prepareNewOpenCommand calls resolveDevice) before validatePreparedOpenRequest runs in prepareResolvedOpenCommand. That means invalid relaunch inputs such as open --relaunch https://... or Android APK targets can be masked by DEVICE_NOT_FOUND when the device is unavailable, instead of returning the expected deterministic INVALID_ARGS error about the bad relaunch target.

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